Engineered photochromism in crystalline salicylidene anilines by facilitating rotation to reach the colored trans -keto form

Article abstract of DOI:10.1021/cg500762a

The photochromism of crystalline salicylidene anilines regularly occurs by a volume-conserving bicycle pedal motion that transposes the relative position of the two atoms of the central imine bond while leaving the original salicyladehyde and aniline rings unchanged. Considering the challenges involved in the preparation of packing structures that are conducive to the bicycle pedal process, we tested a design based on a structural strategy that is known to facilitate the rotation of phenyl rings in the solid state, allowing the molecules to be photoreactive. Three salicylidene aniline molecular rotors linked to bulky trityl, tetraphenylmethyl, or pentiptycene stators were prepared and crystallized, and their solid state photochromism was confirmed. In one case, isomorphous crystals of a deuterium labeled deoxo-salicylidene aniline model system were analyzed by solid state ²H NMR to confirm that the corresponding packing structure is conducive to fast rotation in the solid state.

Full text of DOI:10.1021/cg500762a

Article
pubs.acs.org/crystal
Engineered Photochromism in Crystalline Salicylidene Anilines by
Facilitating Rotation to Reach the Colored trans-Keto Form
Ira O. Staehle, Braulio Rodríguez-Molina, Saeed I. Khan, and Miguel A. Garcia-Garibay*
Chemistry and Biochemistry Department, University of California, Los Angeles, 90095 United States
S
* Supporting Information
ABSTRACT: The photochromism of crystalline salicylidene anilines
regularly occurs by a volume-conserving bicycle pedal motion that
transposes the relative position of the two atoms of the central imine
bond while leaving the original salicyladehyde and aniline rings
unchanged. Considering the challenges involved in the preparation of
packing structures that are conducive to the bicycle pedal process, we
tested a design based on a structural strategy that is known to facilitate
the rotation of phenyl rings in the solid state, allowing the molecules to
be photoreactive. Three salicylidene aniline molecular rotors linked to
bulky trityl, tetraphenylmethyl, or pentiptycene stators were prepared
and crystallized, and their solid state photochromism was confirmed. In
one case, isomorphous crystals of a deuterium labeled deoxo-
2
salicylidene aniline model system were analyzed by solid state H
NMR to confirm that the corresponding packing structure is conducive to fast rotation in the solid state.
INTRODUCTION
■
Photochromism is the reversible transformation of a single
chemical species between two states that absorb in different
regions of the ultraviolet and visible spectrum.1 Photochromic
molecules are of considerable interest due to their potential use
in many new technologies, including nonlinear optics,2
information storage,3 sensors,4 and molecular machines.5
These and other applications require photochromism to
operate in solid matter, that is, crystals, polymers, or thin
films. For that reason, the solid-state photochromism of an
increasing number of compounds like diarylethenes,6 spiropyr-
ans,7 azobenzenes,5,8 and salicylidene anilines9 has been
explored with increasing interest.
Photochromism generally entails changes in bonding
patterns and extent of conjugation resulting from electrocyclic
reactions, proton transfer, and tautomerism, as well as trans-to-
cis double bond isomerizations.1 Photochromism also causes
Figure 1. Photochromism of crystalline salicylidene anilines (SA)
through (a) hypothetical ring rotation in a crystal-engineered
molecular rotator built with a shielding stator that creates free volume
for rotation, and (b) by a volume conserving, bicyle-pedal motion in
the cavity of a close-packed crystal.
changes in size and shape that are generally not allowed within
the rigid environment of a solid matrix. In fact, the detailed
nature of the crystalline environment plays a critical role in
determining the presence or absence of photochromic behavior
for homologous series that consistently display it in solution.10
For example, out of three polymorphs of N-3,5-di-tert-
butylsalicylidene-3-carboxyaniline, only the α and β forms
exhibit photochromism while the structure of the γ form
remains unchanged upon exposure to light.11 In the specific
case of salicylidene anilines (SAs), they exist in thermal
equilibrium between cis-enol and cis-keto tautomeric forms, as
shown in Figure 1, with the aromatic hydroxyl-bearing
tautomer being the most stable.
Photochromism in these molecules occurs upon electronic
excitation of the stable cis-enol followed by an adiabatic oxygen-
to-nitrogen proton transfer reaction and an isomerization
process that positions the N−H group opposite to the carbonyl
oxygen to form the longer-lived, trans-keto species,1a which has
Received: May 25, 2014
Published: June 2, 2014
© 2014 American Chemical Society
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a red colored ortho-quino-dimethane chromophore12 (Figure
1). While the proton-transfer step should not be hindered in a
solid state environment, isomerization from the cis-enol to the
trans-keto structure is expected to be difficult, especially in
close-packed crystals.13 In fact, substantial X-ray structural
evidence suggests that this event occurs by a volume conserving
bicycle pedal motion1a,14,15 involving a reorientation of the
central imine/enamine atoms (Figure 1b). One strategy to
facilitate this process takes advantage of bulky substituents that
twist the aromatic rings out of plane and generate some free
volume in the lattice.15 Considering simple alternatives and
based on our experience in the field of crystalline molecular
rotors,16 we recognized that solid-state photochromism in SAs
may be deliberately enabled by engineering crystals that allow
the full rotation of the hydroxyl-containing ring (Figure 1a). In
analogy with crystalline molecular rotor 117 in Figure 2, which
a
Scheme 1
a
Reaction conditions: (i) 4-bromo-2-hydroxybenzaldehyde, Pd-
(PPh3)4, CuI, diisopropylamine, benzene, reflux 8 h; (ii) 4-bromoani-
line, Pd(PPh3)4, CuI, diisopropylamine, benzene, reflux 48 h; (iii) 4 Å
molecular sieves, toluene, reflux 20 h.
two distinct alkynes the molecule. Accordingly, only two signals
at 84.8 and 98.4 are observed for 3. Two signals at 56.17 and
56.20 ppm correspond to the two quaternary trityl carbon
atoms in the case of 2, while the corresponding values in the
case of 3 are 56.2 and 64.8 ppm. These assignments were
corroborated in the case of 2 by analysis of a 2D HMBC
spectrum. The FTIR spectra of solids 2 and 3 have a very broad
signal that spans from ca. 3400−2600 cm−1 attributed to the
O−H stretch and a signal at ca. 1620 cm−1 assigned to the C
N functionality, in agreement with the cis-enol form.19
As shown in Scheme 2, the synthesis of the bis-pentiptycene
derivative 4 was completed by condensation of the known 6-
Figure 2. Phenylene molecular rotor 1 along with SA modifided rotors
2, 3, and 4 (R = hexyl) with structures intended to facilitate the
rotation of the phenol ring so that it can form the colored trans-keto
form.
a
Scheme 2
has a phenylene rotator shielded by two trityl groups acting as
the stator, we decided to explore analogous structures with the
two SA rings shielded by a bulky framework (Figures 1a and 2).
Using this basic design, we have confirmed the solid state
photochromism of salicylidene aniline-bearing structures 2, 3,
and 4 (Figure 2). Evidence that rotation of the phenol ring is
possible was obtained by measuring the rotational dynamics of
an isomorphous crystal of a d4-labeled Schiff base analogue of 3
where the hydroxyl group had been removed (see below).
a
Reaction conditions: (i) potassium carbonate, 1-bromohexane, dry
SYNTHESIS AND CHARACTERIZATION
■
acetone, reflux 12 h, followed by TBAF, THF at 50 °C for 0.5 h; (ii) 4-
bromo-2-hydroxybenzaldehyde, Pd(PPh3)4, CuI, diisopropylamine,
benzene, reflux 48 h; (iii) 4 Å molecular sieves, toluene, reflux 20 h.
The SA molecular rotors 2 and 3 were obtained by the
convergent strategy shown in Scheme 1. For the synthesis of 2,
samples of tritylacetylene were converted into the salicylalde-
hyde half molecular rotor 5 using commercial 4-bromo-2-
hydroxybenzaldehyde and standard Sonogashira conditions.18
Compound 6 was obtained in a similar manner from the same
alkyne and 4-bromoaniline, and the desired SA 2 was prepared
in 73% yield by condensation of 5 and 6. Compound 3 was
obtained by condensation of commercial 4-amino-tetraphenyl-
methane with SA 5 in 89% yield. The 1H NMR spectra of 2 and
3 have the characteristic signals of the aldimine hydrogens with
chemical shifts of 8.60 and 8.62 ppm, respectively, and those of
the phenol hydrogens at 13.21 and 13.37 ppm. Their
corresponding 13C NMR spectra showed signals of the aromatic
hydroxyl-bearing carbon at 160.8 ppm for both compounds,
and the imine carbons at 161.8 and 161.5 ppm. Four signals in
the range of 84 and 97 ppm in the case of 2 correspond to the
amino-13-hydroxy-pentiptycene20 with salicylaldehyde pentip-
tycene 8 with a yield of 74%, as shown in Scheme 2. This
synthesis started with the preparation of compound 8 by
Sonogashira coupling of commercial 4-bromo-2-hydroxyben-
zaldehyde with the pentiptycene derivative 7. Precursor 7 was
formed in two steps starting from the TMS-protected alkyne
pentiptycene,21 which was alkylated using 1-bromohexane to
1
increase its solubility. The H NMR spectrum of compound 4
showed that the protons on the bridgehead carbons of the
different pentiptycene halves were resolved with chemical shifts
at 5.62, 5.74, 5.75, and 5.94 ppm, each signal integrating for two
1
protons to a total of eight bridgehead protons. All the H and
13C NMR signals from the central SA chromophore were
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consistent with those described above for compounds 2 and 3.
Similarly, in addition to the stretching bands assigned to the SA
unit, the IR spectrum of 4 has two broad phenolic O−H bands
in the 3500−3100 cm−1 region, corresponding to the OH of
the pentiptycene portion and the cis-enol form.
Finally, in order to explore the rotational dynamics of the
trityl group-shielded SA phenolic ring (ring A in Figure 1), we
synthesized d4-labeled compound 10 by condensing 4-amino-
tetraphenylmethane with deuterated aldehyde 9 in 62% yield
(Scheme 3). The spectroscopic properties of this model
a
Scheme 3
a
Reaction conditions: (i) 4-bromo-2-hydroxybenzaldehyde, Pd-
Figure 3. (a) Molecular structure of SA molecular rotor 2 showing a
superposition of two disordered positions with occupancy of 56:44
and the central phenyl rings adopting a twisted conformation. (b) One
of the two disordered positions with occupancy 87% of 3 that
crystallizes with one cyclohexane molecule (not shown). (c) Molecular
structure of the solvate of pentiptycene SA molecular rotor 4 obtained
from acetonitrile (solvent molecules omitted). All the thermal
ellipsoids are shown at 50% probability.
(PPh3)4, CuI, diisopropylamine, benzene, reflux 8 h; (ii) 4 Å molecular
sieves, toluene, reflux 20 h.
compound were in good agreement with expectations,
including the degree of deuterium labeling, which also was
confirmed by high-resolution mass spectrometry. Satisfyingly,
as described below, the powder X-ray diffraction of imine 10 is
essentially identical to that of the analogous SA rotor 3, which
is indicative of a nearly identical packing structure.
occupancy 56:44. As a result of the disorder, a confirmation of
the cis-enol previously determined by spectroscopic means was
not possible. The CAr−OH and the CAr−H bond distances
displayed an average 1.289(2) Å, instead of values of ca. 1.35
and 0.95 Å expected for phenolic and C−H bonds,
respectively.14a The Schiff base has a conformation where the
two aromatic rings in the salicylidene aniline portion are not
planar, with the angle between mean planes slightly larger than
45°; this twisted conformation is characteristic of compounds
with photochromic behavior,22 which in the case of compound
2 was explored by diffuse reflectance in the solid state, as
discussed in the following sections. The packing structure
shows the lattice-forming trityl groups guiding the crystal-
lization in a displaced parallel arrangement by adopting a Ph···
Ph interaction known as a 4-fold phenyl embrace or quadruple
phenyl embrace (QPE),23 with two phenyl rings showing edge-
to-face propagating interactions with neighboring trityl groups.
Some interdigitation occurs between adjacent molecules such
that the trityl group of adjacent molecules are in close proximity
with the salicylidene aniline component (Figure 4a).
Crystals of SA molecular rotor 3 were obtained from a
mixture of cyclohexane/dichloromethane. Small single crystals
had a tendency to degrade when exposed to ambient
conditions. X-ray diffraction data was solved in the monoclinic
system, space group C2 with one molecule of cyclohexane per
molecule of 3. Structural analysis was complicated by the
disorder of the salicylidene aniline part over two alternate
positions related by 180° over an axis perpendicular to the
molecular axis with an occupancy of 87:13. In our model, only
the major component was refined anisotropically and is shown
in Figure 3b. In this structure, the central aromatic rings adopt a
CRYSTALLIZATION STUDIES AND SINGLE
CRYSTAL X-RAY DIFFRACTION ANALYSES
■
Crystallization studies for compounds 2−4 were carried out at
room temperature using different dry solvents and solvent
mixtures. The selection of solvents was based on sample
solubility and on the stability of the imine groups. Samples were
soluble in chloroform, dichloromethane, toluene, and xylenes,
but their solubility was low in acetonitrile and in hexanes.
Traces of water in methanol and ethanol caused the slow
hydrolysis of 2, and these solvents were avoided. In a typical
experiment, ca. 15 mg of the selected compound was placed in
a small glass vial, dissolved with the appropriate solvent or
solvent mixture (ca. 1 mL), and capped. Samples were
monitored regularly until the appearance of the first crystals,
which were taken out of the solution and immediately covered
with oil to prevent the escape of solvent of crystallization. X-ray
data was collected at 100(2) K, and selected crystallographic
parameters for all three compounds are compiled in the
Supporting Information.
Small crystals of 2 were obtained by slow evaporation from
dichloromethane, and their structure was solved in the triclinic
space group P1 with one SA molecular rotor and one highly
̅
disordered solvent molecule per asymmetric unit. The crystals
became opaque when exposed to open air, suggesting a rapid
desolvation process. The molecular structure showed disorder
in the salicylidene aniline portion, which was particularly
evident in the C−OH and CN groups, which were
disordered over two positions related by a 180° rotation over
an axis perpendicular to the molecular axis (Figure 3) with an
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trans-keto forms of SA molecular rotors 2−4, spectroscopic
measurements were carried out using UV−vis diffuse
reflectance before and after exposure to UV light.
Well-characterized polycrystalline powders were placed onto
a barium sulfate plate and pressed, and their diffuse reflectance
was measured before irradiation with 365 nm UV light for 15
min at 300 K.25 After that, as illustrated in Figure 5 for the case
Figure 4. Comparison of the crystal packing of compounds 2 and 3
showing the parallel array of molecules along close contacts (light blue
lines) with the salicylidene aniline portion (highlighted in red). (a)
Compound 2 showing four intermolecular contacts of the adjacent
trityl groups over the aniline ring and two over the salicylidene ring.
(b) In compound 3, the aniline portion has three close contacts, two
of them with the loosely bound solvent highlighted in pink, while the
salicylidene ring has two (one with solvent).
Figure 5. Optical absorption from the Kubelka−Monk function
analysis of diffuse reflectance measurements of SA rotor 3 obtained
immediately after irradiation with λ = 365 nm (orange arrow) and after
various decay periods (blue arrow).
of compound 3, diffuse reflectance measurements of the
photogenerated trans-keto form were carried out over time
scales that ranged from 6000 (ca. 1.6 h) to 50000 s (ca. 13.9 h).
While a photochromic response was observed for all three
samples, their increase in the visible absorption band was
different; in the order 3 > 2 ≫ 4 (Table 1). The spectrum of
more planar conformation with an angle between mean
aromatic planes close to 23°. Packing interactions between
adjacent trityl groups favored a herringbone-like packing of
aromatic molecules, where molecules of compound 3 crystallize
in perpendicular layers with the molecular axis roughly parallel
to the b,c-crystallographic plane. Compared with compound 2,
the salicylidene aniline moiety in 3 shows fewer intermolecular
interactions, mainly with the cyclohexane molecules in the
lattice (Figure 4b).
Table 1. Extent of Reaction, Lifetime of the trans-Keto
Form, and Parameters That May Affect the Photochromism
of the Three SA Rotors
Compound 4 showed the lowest solubility of all, and crystals
obtained from dichloromethane or chloroform become opaque
within seconds when exposed to air. Plates obtained by slow
evaporation from a mixture of dichloromethane/acetonitrile
were suitable for X-ray diffraction. Data were collected at
100(2) K and solved in the monoclinic space group P21/c. A
considerable amount of highly disordered solvent was found
within the cavity generated by the pentiptycene framework,
which is well-known to form host−guest complexes.24 In our
model, five molecules of acetonitrile were found per molecule
of 4. In contrast to the molecular structures of compounds 2
and 3, that of compound 4 did not present disorder of the
salicylidene aniline moiety, with CAr−OH and CN bond
distances of 1.354(3) and 1.287(3) Å, respectively, indicating
the presence of a cis-enol tautomer. The salicylidene rings adopt
a twisted conformation with an angle between mean planes of
45.6°. In this array, the pentiptycene halves acquire a coplanar
conformation, allowing adjacent molecules to aggregate in a
head-to-head fashion forming parallel-displaced layers in a
zigzag pattern (see Supporting Information). The rigid
framework efficiently protects the inner part of the molecule
by generating a relatively large cavity filled with acetonitrile.
SA rotor 2
SA rotor 3
SA rotor 4
1.06
increase factor in keto form
1.69
4.5
a
absorption after hν
lifetime T1 (s)
lifetime T2 (s)
174 14
1392 71
1.271
405
2
1819 472
16468 1920
1.216
1952 13
1.208
density, ρ [Mg m−3
]
near neighbors/number of
6/2
5/2
2/5
b
solvent
a
Equilibration attained after ca. 15 min irradiation at 365 nm.
Number of atoms from neighboring molecules within a threshold of 3
Å. The distances for these close contacts can be found in the
b
Supporting Information.
compound 2 is characterized by an absorption band that
extends from ca. 270 to 450 nm with a λmax at ca. 380 nm that
can be assigned to the cis-enol. A weak shoulder in the range of
450−550 is assigned to the small equilibrium concentrations of
the cis-keto form. Exposure to 365 nm UV light resulted in the
growth of a broad band that extends up to 600 nm with a λmax
at ca. 475, with an absorbance increased by a factor of 1.69
(Table 1) from 0.065 to 0.110 0.003. This band is assigned to
the trans-keto form, which decays in a manner that can be
described by a biexponential function with components having
lifetimes of 174 14 s and 1392 71 s.
SOLID STATE PHOTOCHROMISM
■
1
As mentioned, the H NMR, 13C NMR, and IR spectra are
consistent with the stable cis-enol as the predominant form. In
order to determine the relative contributions of the cis-enol and
The solid state spectra obtained from compound 3 (Figure
5) were similar to those of 2. Before irradiation, the spectrum
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has a λmax at 376 nm and a weak shoulder that extends to ca.
600 nm, which we tentatively assign, respectively, to the cis-enol
and the cis-keto forms. Exposure to UV light resulted in the
growth of a new broad, vibrationally resolved absorption band
between ca. 400−600 nm with a 4.5-fold increase in absorbance
from 0.060 to 0.270 0.002 at 525 nm (Figure 5). The lifetime
for 3 was also fit to a biexponential function, with a fast
component of 405 2 s and a slow component of 1952 13 s.
Solid samples of compound 4 showed a spectrum with a broad
band between 275 and 450 nm and a λmax at ca. 350 nm and
weaker broad band that extends from 450 to 600 nm. Notably,
irradiation with UV light resulted in a very small growth of the
latter with a change in absorbance by a factor of only 1.06, from
Excellent simulation models developed by experts are currently
available on the Internet.30,31 In the case of deuterated
phenylenes, we consider (1) a quadrupolar coupling constant
QCC = 180 kHz, (2) an asymmetry parameter η = 0, (3) the
number of sites along a symmetric potential, which in this case
is n = 2, (4) a value of α = 60° for the cone angle made by the
rotating C−D bond vector and the rotational axis, and (5) the
frequency of exchange between sites.28,29 Spectral changes
display a relatively high sensitivity in the intermediate regime
between ca. 104 and 108 Hz. Slow exchanging (<104 Hz) and
static samples are characterized by a symmetric spectrum with
two intense peaks separated by ca. 130 kHz that are flanked by
two outer shoulders that extend up to ca. 250 kHz.28,29 By
contrast, the spectrum observed for rotational exchange in the
fast exchange limit (>108 s−1) consists of two peaks with a
separation of only 32 Hz, tall shoulders that expand a width of
ca. 125 kHz, and a second set of small peaks separated by ca.
155 kHz. Intermediate rotation frequencies lead to spectra
whose appearance is between these.
0.240 to 0.255
0.005. The lifetime for 4 was fit with a
biexponential function, with a fast component having a lifetime
of 1820 470 s and a slow component having a lifetime of
16470 1920 s. The long wavelength absorbance and small
photochromic response in the case of 4 is likely the result of
electron donation by the hydroxyl group.26
A potential correlation between crystal density or number of
close neighbors in the lattice and the magnitude of the
photochromic response observed is not evident. A clear
distinction between compounds 2 and 3, which have reasonably
high responses, and compound 4, which shows a very small
variation, indicates the importance of electronic factors. It is
worth noting that the lifetime of the trans-keto tautomers of
compounds 2 and 3 are relatively short compared with those of
other structures, in agreement with a molecular design intended
to facilitate the rotations of the hydroxyl-bearing aromatic
group.15c,27
2
As illustrated in Figure 6, H NMR spectra obtained in the
temperature range between 270 and 348 K display changes in
2
SOLID-STATE H NMR
■
In order to support the suggested role for molecular rotation on
the solid state photochromism of salicylidene anlines (Figure
1), we analyzed the rotation of a model Schiff base prepared
with aldehyde analogue 9, which lacks the ortho-hydroxyl
group. Considering the relatively large photochromic response
observed in the case compound 3, we prepared analogue 10
with a deuterium-labeled aldimine ring (Scheme 3). We
reasoned that removal of the hydroxyl group would be likely
to result in a crystalline solid with a packing arrangement
isomorphous to that of 3. Gratifyingly, X-ray powder diffraction
data corroborated this hypothesis (see Supporting Informa-
tion), which allowed us to confirm that the shielding provided
by the trityl framework would allow for the fast rotation of the
aldimine. Without energetic contributions from the intra-
molecular hydrogen bond, we expected the rotational potential
to be symmetric with equivalent sites related by 180° jumps.
We selected a d4-labeled aldimine derivative because solid-state
2H NMR is an ideal technique to probe the solid state dynamics
of groups bearing C−D bonds.28 Quadrupolar echo spectra
acquired with static powdered samples as a function of
temperature (ca. 100 mg) are expected to display changes in
line shape that depend on the frequencies and trajectories of
motion. In the case of 1,4-phenylene groups undergoing 180°
rotations in crystalline environments, a very broad spectrum
characteristic of static samples, known as a Pake pattern, can be
obtained at sufficient low temperature.29
2
Figure 6. Experimental (left) and simulated (right) H line shapes of
model compound 10. The experimental data was obtained at the
temperatures indicated, and the simulated spectra were obtained with
the program NMRweblab using the rotational exchange frequencies
noted and the parameters listed in the text.
line shape with the intensity of the outer peaks decreasing and
that of the inner peaks increasing, as expected for rotational
motion within the 104 to 107 Hz range. The general features of
the spectra and the changes observed as a function of
temperature above 295 K were reasonably matched with a
simulation model that assumes a Log-Gaussian distribution of
rotational frequencies with a width σ = 1.0 centered in the
range of 1.2−10 MHz.31,32 A Log-Gaussian distribution is
indicative of a Gaussian distribution of activation energies,32
which is consistent with the orientationally disordered nature of
the sample. Notably, the spectrum obtained at 270 K is poorly
matched by a simulation carried out with values extrapolated
from the other five data points and the frequency suggested
should be considered as an upper limit. Although this
disagreement may be due to the greater uncertainty of the
data near the slow exchange regime, it may also arise from a
temperature-dependent potential energy surface related to the
Changes in the spectral line shape occur in predictable
manner as a function of rotational exchange rates as the
temperature increases. In practice, the experimental spectrum is
simulated using a suitable dynamic model that includes a set of
parameters that are characteristic of the molecules of interest.
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“softening” of the crystal within the corresponding temperature
range.33 Measurements at temperatures higher than 350 K did
not cause significant changes in the spectra, indicating that
rotation occurs with rates that are faster than the time scale of
2
the H NMR experiment (fast exchange regime).
Using the experimental temperatures and the mean exchange
frequencies of the simulations, we were able to construct an
Arrhenius plot to determine the approximate activation energy
(Ea) and pre-exponential factor (A) (Figure 7). While a value of
Figure 8. (a) Crystal structure of cis-enol 3 in blue superimposed on
the trans-keto via 180° rotation in orange. (b) Crystal structure of cis-
enol 3 in blue superimposed on the trans-keto via bicycle pedal
functionalized to emulate 3 in yellow.
Figure 7. Arrhenius plot of 10 indicating a mean barrier to rotation of
11.2 kcal in the crystalline state.
reorientation of the central CN double bond but also a
change in the direction of the two aromatic rings, which
requires some motion of the trityl groups that are likely to act
as anchors and prevent this motion. The RMSD35 calculated for
the ring rotation mechanism is only 3.3 × 10−5 and that for the
bicycle pedal motion is 0.67. Both these values and inspection
of Figure 8 indicate that the bicycle pedal mechanism requires
significantly greater structural changes and should be more
demanding than the 2-fold ring flip, supporting the latter as the
preferred mechanism.
Ea = 11.2 kcal mol−1 is in line with our design and expectations
for the phenylene rotation, a pre-exponential factor A = 1.5 ×
1014 s−1 is greater than that expected for a phenylene rotator
based on its moment of inertia.17 Notably, computational
analysis of a model (E)-N-benzylideneaniline carried out at the
B3LYP and M06 levels of theory by rotation of the phenyl−
aldimine σ bond (PhCN−) while the rest of the structure
is allowed to minimize suggested barriers to rotation of 8.43
and 8.76 kcal/mol, respectively. These values suggest that the
experimental barrier in the case of 10 may be largely
determined by molecular electronic factors, rather than by
steric barriers in the crystal lattice. Thus, recognizing that
aldimine 10 is isomorphous with SA 3, it is reasonable to
conclude that rotation of the oxygen-bearing ring should not be
significantly inhibited by nearest neighbors.
CONCLUSIONS
■
We have synthesized and fully characterized three molecular
rotors with salicylidene aniline rotators encapsulated by bulky
groups that generate a low density region for rotation of the
oxygen-bearing aromatic group to rotate in the solid state.
While all three SA molecular rotors generated the colored trans-
keto tautomer upon irradiation with UV light, the extent of the
photochromic reaction was significantly larger for the simpler
SA chromophores 2 and 3 compared with the para-hydroxy
substituted chromophore in molecular rotor 4, which highlights
the importance of electronic factors. In order to obtain
evidence for a mechanism that involves the postulated ring
rotation, we prepared and analyzed the rotational dynamics of
molecular rotor 10, an isotopically labeled deoxo-analogue of 3
REACTION CAVITY ANALYSIS
■
Reactions in crystals require that the size and shape of the
transition states, intermediates, and final products match well
those of the reaction cavity determined by the boundary
formed by the nearest neighbors of the reactant.34 The relative
preference of two competing products can be determined by
measuring the root-mean-square deviation (RMSD) of the best
overlap between each of their structures and that of the
reactant, which is a measure of how well they would fit in the
reaction cavity. Considering that the trans-keto forms in Figure
1 originating either from ring rotation or bicycle pedal motion
are actually different structures, we decided to analyze the
RMSD of their best overlap with the structure of the cis-enol, as
shown in Figure 8. The structure of the trans-keto form was
modeled by simple rotation of the oxygen-bearing ring. In
contrast, the structure corresponding to the bicycle pedal
mechanism was modeled by taking the coordinates of the
salicylidene aniline portion of the crystal data for the trans-keto
form shown by Harada et al. to occur by this mechanism14a and
attaching the bulky trityl and tritylethynyl groups. It has been
shown that bicycle pedal motion requires not only the
2
that crystallizes in an isomorphous crystal stucture. Using H
NMR line shape analysis, we determined the ground state
barrier for rotation of 10 to be 11.2 kcal/mol. Furthermore, a
very small RMSD value for the best structural overlap between
the starting cis-enol and the trans-keto form obtained by ring
rotation suggest allowed solid state reactions. The evidence
presented here strongly supports the use of crystal-engineered
ring rotation as an alternative to bicycle pedal motion to obtain
photochromic salicylidene anilines.
ASSOCIATED CONTENT
* Supporting Information
Synthesis, analytical data, and experimental procedures and
crystallographic data (CIF) for salicylidene aniline molecular
■
S
3672
dx.doi.org/10.1021/cg500762a | Cryst. Growth Des. 2014, 14, 3667−3673

Crystal Growth & Design
Article
Hawthorne, M. F.; Garcia-Garibay, M. A. Proc. Natl. Acad. Sci. U. S. A.
rotors 2, 3, and 4. This material is available free of charge via
the Internet at http://pubs.acs.org.
2010, 107, 14973. (c) Khuong, T.-A. V.; Nunez, J. E.; Godinez, C. E.;
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Garcia-Garibay, M. A. Acc. Chem. Res. 2006, 39, 413−422. (d) Garcia-
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A. J. Am. Chem. Soc. 2002, 124, 2398. (b) Dominguez, Z.; Dang, H.;
Strouse, J. M.; Garcia-Garibay, M. A. J. Am. Chem. Soc. 2002, 124,
7719.
AUTHOR INFORMATION
Corresponding Author
*E-mail: mgg@chem.ucla.edu.
■
Author Contributions
The manuscript was written through contributions of all
authors.
́
(18) Chinchilla, R.; Najera, C. Chem. Soc. Rev. 2011, 40, 5084.
(19) These samples were checked before and after irradiation;
however, no additional IR stretches were observed.
Funding
(20) Yang, J.; Ko, C. J. Org. Chem. 2006, 71, 844.
This research was supported by NSF Grant DMR1101934.
Notes
The authors declare no competing financial interest.
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